Plasma propulsion device having special magnetic field



June22, 1965 w. R. BAKER ETAL PLASMA PROPULSION DEVICE HAVING SPECIALMAGNETIC FIELD Filed Sept. 20, 1962 2 Sheets-Sheet l INVENTORS WILLIAMR. BAKER KLAUS HA LBA CH W. LAYMAN BY I ROBERT /Md-0 A TTORNE Y J1me71965 W. R. BAKER ETAL PLASMA PROPULSION DEVICE HAVING SPECIAL MAGNETICFIELD Filed Sept. 20, 1962 2 Sheets-Sheet 2 .Eumtao TIME .rzmmmDo TIMEwzm mma EM ES TIME INVENTORS WILLIAM R. BAKER y KLAUS HALBACH ROBERT W.LAYMAN ATTORNEY United States Patent 3,191,092 PLASMA PROPULSIDN DEVICEHAVING SPECIAL MAGNETIC FIELD William R. Baker, Orinda, and KlausHalbach and Robert W. Layman, Berkeley, Calif., assignors to the UnitedStates of America as represented by the United States Atomic EnergyCommission Filed Sept. 20, 1962, Ser. No. 225,169 6 Claims. ((31.315-111) The present invention relates generally to magnetohydrodynamicdevices for creating energetic ion-electron plasmas and moreparticularly toapparatus for generating and ejecting a high velocitydirected plasma beam. The invention may variously be utilized as apropulsion system for space vehicles and as a stationary injector forplasma containment apparatus.

In copending US. patent application Serial No. 37,816, filed June 21,1960, now issued as US. Patent No. 3,021,272 on February 13, 1962, andapplication Serial No. 112,132, filed May 23, 1961, now issued as US.Patent No. 3,096,269 on July 2, 1963, devices for creating, heating andcontaining a pure plasma are described. In particular, the class ofapparatus described in the aboveidentified copending applicationsproduces a pure plasma from a quantity of a suitable neutral gas. Apulse of the gas is injected into an evacuated housing containingcrossed electric and magnetic fields, the apparatus having aconfiguration in which contact of the plasma with electrical insulatorsor other surfaces is temporarily avoided to prevent the release ofimpurities into the plasma. Both Larmor heat energy in the form ofindividual charged particle orbiting and drift energy in the form ofoverall plasma motion is obtained.

The present invention utilizes a related mechanism for generating plasmain combination with unique means for efliciently converting both theLarmor and the rota tional drift motion of the plasma into a linearlydirected motion to eject high velocity plasma from the apparatus.

In utilizing a plasma device for rocket propulsion, the energy expendedin the gas ionization process should be relatively low compared to theenergy used in unidirec tional acceleration as only the latter producesuseful thrust on the space vehicle. A gas with low ionization energyrequirements such as argon is therefore preferred so that the majorportion of input energy is employed in producing thrust. The specificimpulse of the plasma propulsion device can be adjusted to give optimumutilization of the energy available from an associated high densitypower source such as a nuclear power plant. The ejected plasma iselectrically neutral, thus the net electrical charge on the spacevehicle is maintained constant, an important consideration in space.

In a preferred form, the invention employs a cylindrical electricallyconductive housing with one open end which is'conical and flaredoutwardly. An axial magnetic field is created through the cylinder andfollows the general configuration of the cone at the outlet end thereof.A radial electric field is established between the straight portion ofthe housing and a long axial central electrode therein. A working gasoutlet valve is disposed near the end of the center electrode, whichvalve releases gas in a short burst or puff. The device can act as itsown switch in that a discharge can occur through the gas between thecenter electrode and the housing when the gas diffuses outwardly fromthe electrode and contacts the housing. The gas is rapidly ionized bythe discharge to form an ion-electron plasma.

The above described crossed magnetic and electric fields cause theplasma to acquire drift energy, the plasma rapidly rotating around thecenter electrode, much as the current carrying conductors in anelectrical motor rotate.

Elhlflhl Patented June 22, 1965 The crossed magnetic and electric fieldsalso cause each plasma particle to circulate in an individual Larmororbit. Up to one-half of the energy delivered to the plasma can be inLarmor motion of the particles, most such energy being in the orbitingions which are much heavier than the electrons. The electrons within theplasma are more significantin maintaining a neutral space charge in theplasma than in energy and thrust considerations.

The Larmor energy in the plasma at the completion of the ionizationprocess may be varied from nearly zero to a value equal to the driftenergy by controlling the intensity of the electric field at the timethe plasma ions are formed from the cold neutral gas. With a low initialelectric field, the Larmor orbits are very. small and the Larmor energyis low. The rotational or drift motion of the plasma can subsequently beincreased to any desired value by then raising the applied electricfield in a time which is long compared to the ion Larmor period. With aninitially high intensity electric field the radius of the Larmor orbitsis much greater and the Larmor energy is much higher.

The Larmor rotational and drift rotational forms of motion are eachredirected to impart an axial motion to the plasma. The plasma driftenergy is utilized to provide an axial plasma motion by causing largeradial currents to fiow through the plasma following a sudden reductionof the rotational motion of the plasma body. This effect is produced byproviding for a short circuit across the rotating plasma, the plasmabeing impelled away from the short circuit by the magnetic field of theintense short circuit current. The short circuiting action may beobtained either by an external switch which operates to connect thehousingand center electrode or by internal current shortening meansdisposed adjacent the plasma containing region in position to becontacted by the axially expanding plasma.

In addition to the axial plasma motion produced as described above, theorbital Larmor motion of plasma particles is also converted to an axialmotion by ejecting the plasma through a diverging magnetic field whichforms a single magnetic mirror, the motional conversion being made bythe magnetic mirror effect. Since each particle of the plasma has twodifferent types of rotational energy, the two coacting mechanisms arerequired for converting such energy to obtain axial motion.

Where the device is to be utilized as a stationary source of heatedplasma for injection into, for instance, a thermo nuclear power device,the magnetic mirror may be reduced or eliminated so that Larmor heatenergy is retained in the plasma. In this instance, only the driftenergy is utilized to impart an axial motion to the heated plasma.

Accordingly, it is an object of this invention to provide a plasmagenerator capable of efficiently ejecting a high velocity directedplasma body.

Itis an object of this invention to provide a magnetohydrodynamic devicein which the kinetic energy of component particles in a plasma isefiiciently utilized to produce directed linear motion of the plasmabody.

It is another object of the present invention to provide a highlyeflicient meansfor converting rotational drift energy of a plasma bodyinto axial motion thereof by providing a short-circuit across thecounter electromotive force produced by the rotating plasma body.

It is an object of the present invention to provide a I It is anotherobject of the present invention to provide a highly efiicientmagnetohydrodynamic means for propelling space craft outside theterrestrial atmosphere.

It is another object of the present invention to provide a plasmainjector in which the ejection of plasma is eltected by converting therotational energy of a plasma body into linear motion of the body.

The invention together with further objects and advantages thereof willbe better understood by reference to the following specificationtogether with the accompanying drawing of which:

FIGURE 1 is a broken-out axial section view showing a plasma propelledrocket engine with associated electrical circuitry, fuel supplies andcontrol elements in block form,

FIGURES 2a to 2e illustrate progressive stages in the operation of theapparatus of FIGURE 1, and

FIGURES 3a to 3e are graphs of plasma current corresponding to thestages of operation shown in FIGURES 2a to 26 respectively.

Referring now to FIGURE 1, there is shown a plasma rocket motor 11having a cylindrical plasma housing section 12 made from an electricallyconductive material. The plasma ejection end of the cylinder 12 isextended by an outwardly flared section 13 made of a similar con ductingmaterial. Flared section 13 has a length exceeding that of thecylindrical section 12 and has a maximum diameter approximately twicethat of the cylindrical section. The forward end of the cylinder 12 isextended for a short distance by an annular insulator 14 of likediameter which is attached thereto in coaxial relationship therewith. Adisc 18 having a central aperture 22 closes the forward end of theinsulator 14 and is secured to an inwardly directed lip 19 on theinsulator 14.

A tubular central electrode 21 is disposed through the aperture 22 inthe disc 18 along the axis of the cylinder 12 and projects alongapproximately three-fourths of the length thereof. A plurality ofapertures 23 are situated in a band encircling electrode 21 at anintermediate longitudinal position thereon near the center of thecylinder 12. Such apertures 23 provide an outlet through which gas maybe emitted into the plasma chamber 24 formed by the cylinder 12. Thedistance between the apertures 23 and the insulator 14 is sufiicientlygreat that ionization of the gas will occur before the gas difiuses tothe insulator, thus preserving plasma plurity. A valve 26 is disposedwithin the central electrode 21, at the apertures 23, and is of a typewhich releases a metered quantity of gas through the apertures in a veryshort time. A suitable structure for valve 26 is described in theaforementioned copending patent applications Serial Nos. 37,816 and112,132, now US, Patent Nos. 3,021,272 and 3,096,269, respectively. Agas supply 28 is coupled to the center electrode 21 and provides gas tothe valve 26 for release in repeated measured bursts through apertures23 as described above.

An axially directed magnetic field is created through the chamber 24 bya plurality of annular magnet coils 29 disposed coaxially around theouter surface of the cylinder 12 and the insulator 14. The magneticfield is slightly less intense in the region of the gas emissionapertures 23, the magnet coils 29 having fewer turns in such locality asindicated by a decrease in the thickness of the coils at such region.Thus the magnetic field lines 32 bow slightly outwardly around theapertures to form a small magnetic mirror 35 thereat. A conical portion31 of the magnet coil is disposed around the outer surface of the flaredsection 13 of the plasma housing so that the resultant magnetic fieldlines 32 follow the approximate shape of the walls of the cylinder 12and flared section 13, with a magnetic mirror being formed in the flaredsection 13. A magnet power supply 33 provides current to the magnetcoils 22 and 31.

A. radial electric field is created between the central electrode 21 andthe cylinder 12 by connections 36 to a high voltage, high current powersupply 34. Although only one set of the connections 36 is shownschematically in FIG- URE 1, it is preferable that many duplicateconductors 36 be distributed around the cylinder 12 and disc 18 so thata symmetrical current flow is provided to the apparatus.

In operation and with reference to FIGURES 2a to 22 and FIGURES 3a to32, the magnet power supply 33 and high current power supply 34 areenergized throughout the entire cycle of operation so that steady statemagnetic and electric fields are provided in the plasma chamber 24.Similarly, it will be understood that the apparatus is situated in avacuum such as is encountered beyond the terrestrial atmosphere and thatthe engine 11 and associated components will generally be mounted in asuitable nacelle structure which may take various forms according to theusage which is to be made of the thrust generator.

Referring now to FIGURES 2a and 3a in particular, thrust generation isinitiated by a momentary opening of the valve 2d, thereby releasing gas51 through the apertures 23 in the central electrode 21. The gas 51 isnot ionized at this stage since electrical contact, through the gas,between the central electrode 21 and the cylinder 12 has not yet beenestablished and thus there is no current flow. In FIGURES 3a to 3e thereis shown a waveform 69 of current through the plasma with the time axisalong the abscissa and plasma current magnitude along the ordinate, theorigin being at time T when the valve 26 is opened. Thus as shown inFIGURE 3a in particular, at portion '71 of waveform 69, no current haspassed through the plasma at the stage of operation shown in FIGURE 20.

Referring now to FIGURES 2b and 3b, a discharge occurs through the gasbetween the cylinder 12 and central electrode 21 after the gas hasdiffused outwardly to the cylinder. It is desirable that all the gas beionized very quickly and to further this objective, the Weak magneticmirror system 35 is provided to concentrate the plasma in one areaduring the initial ionization stage. The first ions and electrons thatare formed serve to ionize many other gas particles by collision itretained in the gas filled area. The magnetic mirror is useful inpreventing the escape of the first ions and electrons and thus causesthe initial ionization operation to occur more rapidly. The ionized gasparticles form a plasma 52 in which the electrons and the ions areattracted in opposite radial directions owing to the radial electricfield between the electrode 21 and cylinder 12. However, the chargedparticles of the plasma are deflected by the magnetic field 32 and arecaused to rotate around the central electrode 21 in the same direction.Simultaneously, the ions and electrons also rotate in smaller Larmororbits within the larger mass plasma motion. At the moment that a gasparticle is ionized, it is impelled through the crossed electric andmagnetic field in a cuspate Larmor orbit. The Larmor motion of theplasma particles is equivalent to an increase in plasma temperature.

After the increase in plasma energy, the magnetic mirror 35 can nolonger restrain the ions and electrons as during the earlier stage. Atthis time, and as indicated in FIGURE 31;, the current through theplasma 52 from the power supply 34 is rising rapidly as shown at point72 on the waveform 6%.

Subsequent to the foregoing stage of operation, the plasma 52, asindicated in FIGURES 2c and 3c, expands along the magnetic field lines32 in both directions from the magnetic mirror 35, while continuallyrotating about the central electrode 21. The plasma current has risen toa peak and is decreasing toward zero value at portion '73. The strayinductance in the power supply 34 and in the connections to the cylinder12 and the central electrode 21 together with the capacitance in theplasma chamber elements combine to form a resonant circuit which causesthe current waveform to be sinusoidal.

bar across the entire rotating plasma.

The current through the plasma 52 from the power supply 34 creates amagnetic field in addition to the steady state axial magnetic field ofcoils 29. The magnetic field associated with the plasma currentgenerates an outwardly directed force that pushes the conductors of thecurrent into as large a loop as possible. Thus there is a magnetic forceon the plasma 52., which is functioning as a conductor, which pushes theplasma toward the open end of the housing 12. The plasma constitutes theonly portion of the current path 53 which may be moved by the magneticfield. However, the magnetic force is not suflicient, at the stage ofoperation shown in FIGURE 20, to keep some of the plasma from driftingtoward the disc 18 at the forward end of the housing 12. The electrons,being more mobile than the ions, move faster and soon contact the disc18. Since electrical conductivity along the magnetic field lines throughthe plasma 52 is quite high, the contact of one end of the plasma withthe disc 18 is equivalent to placing a radial short circuit or crow- Itis generally preferred that the short circuit occur at the same momentthat the current 53 through the plasma 52 reaches a zero value. Suchshort circuit has the effect of decoupling trons.

the power supply 34 from the plasma 52. 'Since'the short circuit appearsacross the rapidly rotating plasma 52, the plasma rotation is halted ina very sudden oscillatory manner, and alternately turns backward andforward, the effect being analogous to the behavior of the rotor of aconventional electric motor upon being shorted.

Referring now to FIGURES 2d and 3d, after the plasma has been shortcircuited as described above, the rotational kinetic energy of theplasma is converted into a magnetic field e ergy by a current pulse 74which flows through the plasma in a direction opposite to that ofthepreviously applied power supply 34 current, such current pulse 74resulting fromthe sudden halting of the plasma rotation. The intensecurrent pulseid creates a magnetic field which 1 acts to eject theplasma 52 through the open end of the engine 11. In FIGURE 2d, the pathof the shorted plasma current is indicated by dashed lines 54 and asshown the plasma 52 is now forced into the flared section 13 of theengine by the expanding magnetic field of the current 54.

. 6 tions repetitively so that the above described sequence ofoperations is repeated with as high a frequency as is per mitted by theparameters of the particular embodiment. In a unit having a diameter ofsix inches with an electrode having a diameter of two inches, fourpounds of thrust are obtainable with 1000 pulses per second and with amagnetic field of 20-30 kilogauss; an electric field of 20- 30kilovolts; and 30 micron liters of argon gas emitted each pulse.

The time interval between ionization of a gas molecule and theconversion of the Larmor motion of the resultant ion to unidirectedmotion should be small enough so that excessive ion energy is not lostby collisions with elec- To permit longer times, the Larmor motion canbe reduced and the drift motion maximized by providing a low initialelectric field during the ionization phase and subsequently increasingthe electric field to give the desired drift velocity. The desiredchange in electric field intensity can be accomplished very efiicientlyby means of an optimum value inductance connected in series with theengine 11 and power supply 34. Since the engine 11 appears electricallyas a capacitor, the effect can be considered as resonance charging.

As a further variation, and with reference again to FIGURE 1, the disc18 at the forward end of plasma chamber 24 may be formed of insulativerather than conductive material and will still function to short circuitthe plasma current as hereinbefore described. Theplasma electrons whichbombard the surface of the insulator convert it into a short circuitwhich acts in much the same manner as in the case of a conductive disc.Such a system is advantageous if the current in the magnetic field coilsis pulsed rather than steady state, since a pulse magnetic field willnot readily pass through a thick conductor such as the disc 18. However,pulsed field operation can also be utilized in the invention as shown inFIGURE 1 if the disc 18 is made thin enough so that the magnetic fieldcan readily penetrate. Pulsed fields may be desired in some instances inthat the axial expelling force on the sity behind the outgoing plasmabody as described in co- The foregoing effect thus functions to convertthe rotational energy of plasma 52 to a linear motion directed away fromdisc 13. To obtain the maximum plasma ejection velocity, it is desirablethat the energy represented by the Larmor motion of component particlesin the plasma also be converted tolinear motion along the axis of theengine. Such conversion occurs when the plasma 52. enters the flaredmagnetic field in housing section 13 since the field forms a magneticmirror. The magnetic mirror effect, wherein the energy of plasmaparticles rotating in Larmor orbits is utilized to impart an axiallydirected motion to the plasma is well understood in the art and isdiscussed in detail in the text: Glasstone and Lovberg, ControlledThermonuclear Reactions, D. Van Nostrand Co., Inc., 1960, pages 61, 62.Referring now to FIGURES 2e and 32, after several cycles 76 of plasmacurrent, as a result of the backward and forward rotation of the plasma,the ringing or damped current oscillations have a period which isshorter than before the short circuit occurred. However, the periodlengthens again when the plasma current path increases in length as aresult of the outward motion of the plasma. Thus, as shown in FIGURE 2e,the plasma 52 is ejected outwardly from the open end of engine 11through the combined effects of the expanding magnetic field of the 7plasma current and the magnetic mirror in the flared section 13. Bothsuch eifects utilize rotational energy of the plasma particles toproduce the directed motion of the plasma body. After the plasma isejected, the engine 11 is then in condition for a succeeding similarcycle of operation. To provide maximum thrust, the valve 26 func pendingU.S. patent application Serial No. 136,686, filed August 9, 1961, andissued as .U.S. Patent No. 3,069,344 on December 18, 1962. It is notessential that the magnetic lines pass through the end closure disc. Themagnetic lines may curve outwardly and pass through the cylinder wall sothat the wall provides the short circuit means. 7

Further, the parameters of the propulsion device may be selected so thatit is possible to eliminate the weak magnetic mirror field 35 and tosubstitute a slightly divergent magnetic field within the cylindricalsection 12. The plasma particles receive some rearward acceleration inthe cylindrical section and either additional rearward acceleration canbe obtained or the acceleration requirements for the flared section 13can be reduced. A limiting factor in the maximum divergence possible forthe magnetic field in the cylindrical section 12 is that a portion ofreference to usage as a propulsion engine, it will be apparent that theapparatus may also be utilized as a means for injecting heated plasmainto a separate utilization device such as the containment devices usedfor inducing thermonuclear reactions. In such usage it may be preferablethat the Larmor heat energy be retained in the plasma and only the driftenergy utilized to impart an axial motion thereto. This is accomplishedby eliminating the magnetic mirror field 32 from the apparatus bydispensing with the flared housing section 13 and conical coil 31 ofFIGURE 1. Generally, a strong axial force is not required from plasmainjectors so the lessened plasma velocity is not a disadvantage.

While the invention has been described with respect to certain exemplaryembodiments thereof, it will be apparent to those skilled in the artthat numerous variations and modifications may be made within the spiritand scope of the invention, and thus it is not intended to limit theinvention except as defined in the following claims.

What is claimed is:

1. In a magnetohydrodynamic device for producing a directed plasma beam,the combination comprising:

(a) a cylindrical housing disposed symmetrically around an axis anddefining a plasma chamber, a first end of said housing being closed anda second end thereof being open to form a plasma outlet,

(b) means providing a substantially linear longitudinal magnetic fieldthrough said plasma chamber and being characterized in that saidmagnetic field is caused to pass through said housing only at said firstend and pass axially through said chamber and flare outwardly from saidaxis at said plasma outlet,

() a tubular electrode disposed along the axis of said housing, saidelectrode being electrically isolated therefrom, said electrode havingat least one gas emission opening at a median position in said chamber,

(d) an electrical power supply coupled between said center electrode andsaid housing, and

(e) means supplying gas to said electrode for pulset emission throughsaid opening.

2. A plasma generator for ejecting a linearly directed plasma beam,comprising in combination:

(a) an annular housing having a closure at a first end and an opening atthe opposite second end,

(b) means providing a longitudinally directed magnetic field through theinterior of said housing and which field intercepts said housing wallonly at said first end, said field having essentially linear fieldintensity within said housing and forming an outwardly facing magneticmirror in the region of said opening,

(0) a tubular central electrode disposed along the axis of said housingin at least the region of the closed end thereof,

(d) a fast opening repetitively operable valve having an inlet andhaving an outlet communicated with the interior of said housing at amedian region along said central electrode,

(e) a gas supply coupled to said inlet of said valve, and

(f) a high current power supply connected between said housing and saidcentral electrode whereby a radially directed electrical field isestablished within said housing.

3. Apparatus for ionizing and heating a gas and for expelling theresultant plasma with a linearly directed motion comprising incombination:

(a) an annular housing having a central electrically conducting sectionof constant diameter and having a broad opening at one end,

(b) a circular conductor closing the other end of said housing,

(0) a long central electrode disposed axially within said housing andhaving a plurality of apertures only at an intermediate longitudinalposition therein,

((1) a fast opening valve of the type releasing a pulse of gas disposedwithin said central electrode and having an outlet communicated withsaid apertures,

(e) a gas supply coupled to said valve,

(f) a high current electrical power supply connected between saidhousing and said central electrode,

(g) a magnet coil disposed around said housing and producing an axialmagnetic field therein, a major portion of said field passing throughsaid circular conductor and being of essentially constant intensitybetween said housing and said central electrode, said magnet coilincluding means diverging said field at said broad opening of saidhousing to form a single outwardly facing magnetic mirror,

(h) a magnet power supply coupled to said coil.

4. Apparatus as described in claim 3 and further characterized by saidmagnet coil having a reduced number of ampere-turns in the longitudinalvicinity of said apertures of said central electrode whereby arelatively weak magnetic mirror is formed therearound.

5. In a magnetohydrodynamic device, the combination comprising:

(a) a,cylindrical electrically conducting housing having one end closedand the opposite end flared outwardly to form a broad plasma outlet,

(b) a long tubular central electrode disposed on the axis of saidhousing, and having gas emission apertures at a median positiontherealong,

( a gas pp y,

(d) a rapidly functioning valve coupled to said gas supply and disposedat said central electrode for emitting bursts of gas into the axialregion of said housa (e) a high voltage power supply connected betweensaid housing and said central electrode, and

(f) a magnet coil disposed around said housing and providing an axiallydirected field of essentially constant intensity in said housing, saidfield decreasing in intensity at said flared end of said housing, saidfield transecting said housing only at said closed end.

6. In a magnetohydrodynamic apparatus for generating a plasma and forconverting rotational kinetic energy and heat energy of said plasma intodirected motion of said plasma, the combination comprising:

(a) a cylindrical housing having a straight section of generallyconstant diameter adjacent a first end and an outwardly flared sectionadjacent the second end,

(b) a flat short circuiting electrode disposed transversely at saidfirst end of said housing and forming a closure therefor,

(c) an annular magnet coil disposed coaxially around said housing forproviding a substantially axially directed field therein, said coilproviding a slightly more intense magnetic field at the ends of saidstraight section of said housing than in the central portion of saidstraight section and providing a magnetic field in said flared sectionof said housing which has an intensity varying inversely with thediameter thereof, said field transecting said housing only at saidclosed end,

((1) a tubular central electrode disposed along the axis of saidstraight section of said housing, said central electrode having gasemission apertures therein at said central portion of said straightsection of said housing,

(e) means supplying gas to said tubular central elec trode for pulsedemission through said apertures thereof, and

(f) a high current electrical power supply connected between saidhousing and said central electrode.

References Cited by the Examiner UNITED STATES PATENTS 3,025,429 3/62Gow et al 176-6 X 3,069,344 12/62 Post et a1. 176--7 X 3,073,984 1/63Eschenbach et al. 3l323l.5

GEORGE N. WESTBY, Primary Examiner.

1. IN A MAGNETOHYDRODYNAMIC DEVICE FOR PRODUCING A DIRECTED PLASMA BEAM, THE COMBINATION COMPRISING: (A) A CYLINDRICAL HOUSING DISPOSED SYMMETRICALLY AROUND AN AXIS AND DEFINING A PLASMA CHAMBER, A FIRST END OF SAID HOUSING BEING CLOSED AND A SECOND END THEREOF BEING OPEN TO FORM A PLASMA OUTLET, (B) MEANS PROVIDING A SUBSTANTIALLY LINEAR LONGITUDINAL MAGNETIC FIELD THROUGH SAID PLASMA CHAMBER AND BEING CHARACTERIZED IN THAT SAID MAGNETIC FIELD IS CAUSED TO PASS THROUGH SAID HOUSING ONLY AT SAID FIRST END AND PASS AXIALLY THROUGH SAID CHAMBER AND FLARE OUTWARDLY FROM SAID AXIS AT SAID PLASMA OUTLET, (C) A TUBULAR ELECTRODE DISPOSED ALONG THE AXIS OF SAID HOUSING, SAID ELECTRODE BEING ELECTRICALLY ISOLATED 